Conventionally, there has been known an ink jet recording apparatus of the type in which orifices, the number corresponding to the image density, are provided in a number of ink discharge devices for enclosing ink, and a pressure pulse is selectively applied to the ink discharge devices so that ink is jetted from the orifices.
In the apparatus of that type, there have been problems as follows. It is necessary to make the ratio of the size of each orifice to that of the corresponding ink discharge device large in order to keep the orifice from becoming clogged with ink. Accordingly, it is difficult to make the ink discharge devices small in size, it is necessary to make the pitch of the orifices correspondingly large, and it is impossible to set the image recording density high. Further, because of the use of mechanical scanning in applying the pressure pulses, the recording speed is inevitably lowered.
As means solving those problems, there has been proposed the so-called magnetic ink jet method in which magnetic ink is disposed in the vicinity of a magnetic electrode array. An ink discharge state corresponding to the image density is formed by using swell of ink due to the magnetic field, and the magnetic ink is caused to jet toward a recording sheet in an electrostatic field (Japanese Patent Unexamined Publication No. 55-69469). There has also been proposed the so-called plane ink jet method in which a slit-ink reservoir parallel to an electrode array is filled with ink, and the ink is caused to jet toward a recording sheet in accordance with an electric field pattern formed between the electrode array and an electrode opposed to the electrode array through the recording sheet (Japanese Patent Unexamined Publication No. 56-37163). Further, the so-called thermal bubble jet method has been proposed, in which thermal energy is applied to ink so as to abruptly heat the ink to cause film boiling. Bubbles are abruptly formed in the orifices so that the ink is caused to jet from orifices by pressure rising there (Japanese Patent Unexamined Publicaiton No. 55-161664).
In the application of the magnetic ink jet method, however, there has been a problem in that it is necessary to use a mixture of ink with magnetic powder so that the ink is black, and it is difficult to obtain a color picture by printing through superposition of ink. In the application of the plane ink jet method, although improvement in blockage with ink can be made because minute orifices are not necessary, application of a high voltage is required to cause ink to jet, so that it is necessary to drive the electrode array in time division in order to prevent voltage leakage from occurring between adjacent electrodes. Further, in the application of the thermal bubble jet method, there have been problems in that it is necessary to abruptly raise the temperature of the heating elements in order to generate film boiling, so that there is a tendency for the characteristics of the ink to be changed, and the protective layer provided for the heating resistors may be thermally deteriorated.
In order to solve such problems, the present inventors have proposed a thermal-electrostatic ink jet recording apparatus which comprises, as shown in FIG. 6, a head body d constituted of a pair of insulating substrates a and b opposed to each other and having a slit-like space portion c formed therebetween, thermal energy application means e for applying thermal energy to ink in the slit-like space portion c, and electrostatic field formation means q for forming a predetermined electrostatic field between the ink surface and a recording sheet f, so that thermal energy is selectively applied, in accordance with image signals, to the ink and the selectively heated portion of the ink under the influence of the predetermined electrostatic field is caused to jet toward the sheet f.
In the thermal-electrostatic ink jet recording apparatus of the above type, it becomes unnecessary to use magnetic ink, as in the magnetic ink jet method, and therefore it is possible to easily realize color printing through superposition of ink. Further, it becomes unnecessary to cause ink to jet only by means of an electrostatic field, unlike the case of the plane ink jet method, so that it becomes unnecessary to make the intensity of the electrostatic field extremely high. Accordingly, voltage leaks in the vicinity of the ink can be effectively prevented. Furthermore, it becomes unnecessary to cause ink to jet only by means of thermal energy, unlike the case of the so-called bubble jet method, so that the quantity of thermal energy can be reduced to an extent and thermal deterioration of ink can be effectively prevented. Therefore, in the proposed apparatus of the type described above, high speed and high density recording can be carried out while effectively preventing the disadvantages in the various conventional systems.
In such a thermal-electrostatic ink jet recording apparatus as described above, the main portion of the thermal energy application means e is constituted, as illustrated in FIG. 6, by a plurality of heating resistors h provided respectively for picture elements and disposed in the slit-like space portion c at portions near the side edge of the discharge opening. A pair of current conduction electrode i and i are provided on each of the heating resistors h for selectively causing a current to flow to each of the heating resistors h. A switching circuit k is connected to the pairs of current conduction electrodes i and i and includes switching elements j arranged to be opened/closed in accordance with signals from a control device (not shown).
In such a thermal-electrostatic ink jet recording apparatus, therefore, there has been a problem in that it is necessary to connect all the pairs of current conduction electrodes i and i of the respective heating resistors h to the switching circuit k, so that the switching circuit k is complicated and the recording head is high in manufacturing cost. If the picture density required for the thermal-electrostatic ink jet recording apparatus is increased, there is a problem in that it becomes difficult to make the switching circuit k so as to satisfy the above requirement.
Alternatively, therefore, an improved thermal-electrostatic ink jet recording apparatus has been developed in which, as shown in FIG. 7, a conductive layer n is provided on the pairs of current conduction electrodes i and i through an insulating layer m. One of the pair of current conduction electrodes i and i of each of the respective heating resistors h is connected commonly to the conductive layer n through a corresponding through hole p formed in the insulating layer m to thereby simplify the arrangement of the switching circuit k. That is, in the apparatus, one of the pair of current conduction electrodes i and i of each of the respective heating resistors h, along with one of the current conduction electrodes of the other resistors, are commonly bonded to the conductive layer n to maintain the one group of current conduction electrodes i at a common potential. The other of each of the pairs of current conduction electrodes i is coupled to the switching circuit k. Thus, the formation of the switching circuit k can be simplified.
In such an improved thermal-electrostatic ink jet recording apparatus, however, there have been further problems in that it is necessary to provide a pair of current conduction electrodes i and i for every heating resistor h similarly to the thermo-electrostatic ink jet recording apparatus illustrated in FIG. 6, so that it is complicated to make the electrodes with a high density. Further, it is necessary to provide the through holes p in the insulating layer m equal in number to the heating resistors h at the same pitch as the latter, so that the operation of forming the through holes is complicated. Accordingly, the recording head productivity is lowered and the manufacturing cost of the recording head is higher. Further, if the required density of the picture elements is high, it is difficult to satisfy this requirement.